SIVA is an adaptor protein that binds to the cytoplasmic tail of CD27 and GITR receptors of the TNF receptor (TNFR) family. It exists as two alternative splice isoforms, SIVA1 and SIVA2. SIVA1 is longer and contains a death domain homology region (DDHR) with a putative amphipathical helix in its central part. SIVA2 is shorter and lacks the DDHR. Both isoforms contain a B-box-like ring finger and a Zinc finger like domain in their C-termini. Enforced expression of both SIVA1 and SIVA2 has been shown to induce apoptosis (Prasad et al., 1997, Yoon et al., 1998, Spinicelli et al., 2003, (Py et al., 2004). SIVA1 induced apoptosis is suggested to be effected by its binding to and inhibition of the anti apoptotic Bcl-2 family members through its amphipathic helical region (Chu et al., 2005; Chu et al., 2004; Xue et al., 2002). Consistent with the pro-apoptotic role, SIVA is a direct transcriptional target for the tumor suppressors p53 and E2F1 (Fortin et al., 2004). Various point of evidence indicate that SIVA is a stress-induced protein and is up-regulated in acute ischemic injury (Padanilam et al., 1998), coxavirus infection (Henke et al., 2000), and also by cisplatin treatment (Qin et al., 2002), as well as TIP30 expression which induces apoptosis (Xiao et al., 2000). Recently, the common N- and C-termini of SIVA1 and SIVA2, yet not the death domain, have been shown to be sufficient and capable to mediate apoptosis in lymphoid cells through activation of a caspase dependent mitochondrial pathway (Py et al., 2004).
NF-κB-inducing kinase, NIK, (MAP3K14) was discovered (Malinin et al., 1997) in a screening for proteins that bind to the TNF-receptor associated adaptor protein TRAF2. The marked activation of NF-κB upon overexpression of this protein kinase, and effective inhibition of NF-κB activation in response to a variety protein kinase, and effective inhibition of NF-κB activation in response to a variety of inducing agents, upon expression of catalytically inactive NIK mutants suggested that NIK participates in signaling for NF-κB activation (Malinin et al., 1997).
NIK has an in lymphoid organ development (Shinkura et al., 1999). Apart from the contribution to the regulation of the development and function of the immune system, NIK seems also to be involved in the regulation of various non-immune functions such as mammary gland development (Miyawaki et al., 1994). In vitro studies implicated NIK in signaling that leads to skeletal muscle cell differentiation (Canicio et al., 2001), and in the survival and differentiation of neurons (Foehr et al., 2000).
Assessment of the pattern of the NF-κB species in lymphoid organs indicated that, apart from its role in the regulation of NF-κB complex(s) comprised of Rel proteins and IκB, NIK also participates in controlling the expression/activation of other NF-κB species. Indeed, NIK has been shown to participate in site-specific phosphorylation of p100, which serves as a molecular trigger for ubiquitination and active processing of p100 to form p52. This p100 processing activity was found to be ablated by the aly mutation of NIK (Xiao et al., 2001b). NIK in thymic stroma is important for the normal production of Treg cells, which are essential for maintaining immunological tolerance. NIK mutation resulted in disorganized thymic structure and impaired production of Treg cells in aly mice (Kajiura et al., 2004). Consistently, studies of NIK-deficient mice also suggested a role for NIK in controlling the development and expansion of Treg cells (Lu et al., 2005). These findings suggest an essential role of NIK in establishing self-tolerance in a stromal dependent manner. NIK also partakes in NF-κB activation as a consequence of viral infection. Respiratory syncytial virus infection results in increased kinase activity of NIK and the formation of a complex comprised of activated NIK, IKK1, p100 and the processed p52 in alveolus like a549 cells. In this case NIK itself gets translocated into the nucleus bound to p52 and surprisingly, these events precede the activation of canonical NF-κB pathway activation (Choudhary et al., 2005). These findings indicate that NIK indeed serves as a mediator of NF-κB activation, but may also serve other functions, and that it exerts these functions in a cell- and receptor-specific manner.
NIK can be activated as a consequence of phosphorylation of the ‘activation loop’ within the NIK molecule. Indeed, mutation of a phosphorylation-site within this loop (Thr-559) prevents activation of NF-κB upon NIK overexpression (Lin et al., 1999). In addition, the activity of NIK seems to be regulated through the ability of the regions upstream and downstream of its kinase motif to bind to each other. The C terminal region of NIK downstream of its kinase moiety has been shown to be capable of binding directly to IKK1 (Regnier et al., 1997) as well as to p100 (Xiao et al., 2001b) and these interactions are apparently required for NIK function in NF-κB signaling. The N terminal region of NIK contains a negative-regulatory domain (NRD), which is composed of a basic motif (BR) and a proline-rich repeat motif (PRR) (Xiao and Sun, 2000). The N-terminal NRD interacts with the C-terminal region of NIK in cis, thereby inhibiting the binding of NIK to its substrate (IKK1 and p100). Ectopically expressed NIK spontaneously forms oligomers in which these bindings of the N-terminal to the C terminal regions in each NIK molecule are apparently disrupted, and display a high level of constitutive activity (Lin et al., 1999). The binding of the NIK C-terminal region to TRAF2 (as well as to other TRAF's) most likely participates in the activation process. However, its exact mode of participation is unknown.
Recently, a novel mechanism of NIK regulation has gained much attention. This concerns the dynamic interaction of NIK and TRAF3 leading to proteasome mediated degradation of NIK. Interestingly, inducers of the alternative pathway of NF-κB like CD40 and BLyS have been shown to induce TRAF3 degradation and concomitant enhancement of NIK expression (Liao et al., 2004).
There is rather limited information yet of the downstream mechanisms in NIK action. Evidence has been presented that NIK, through the binding of its C-terminal region to IKK1 can activate the NB kinase (IKK) complex. It has indeed been shown to be capable of phosphorylating serine-176 in the activation loop of IKK1 and thereby its activation (Ling et al., 1998).
It was suggested that NIK does not participate at all in the canonical NF-κB pathway, but rather serves exclusively to activate the alternative one (see (Pomerantz and Baltimore, 2002, for review).
Lately, it was shown that although the induction of IkappaB degradation in lymphocytes by TNF is independent of NIK, its induction by CD70, CD40 ligand, and BLyS/BAFF, which all also induce NF-kappaB2/p100 processing, does depend on NIK function (Ramakrishnan et al. 2004). Both CD70 and TNF induce recruitment of the IKK kinase complex to their receptors. In the case of CD70, but not TNF, this process is associated with NIK recruitment and is followed by prolonged receptor association of just IKK1 and NIK. Recruitment of the IKK complex to CD27, but not that of NIK, depends on NIK kinase function. These findings indicate that NIK participates in a unique set of proximal signaling events initiated by specific inducers, which activate both canonical and noncanonical NF-kappaB dimers.
TRAF family in mammals is comprised of seven members TRAF1-TRAF7(Bradley and Pober 2001, Xu et al., 2004). TRAFs play important functions in both adaptive and innate immunity, mainly by the activation of transcription factors NF-kB and AP1 (Wajant and Scheurich, 2004). All TRAF proteins share a C-terminal homology region termed TRAF domain that is capable of binding to the cytoplasmic domains of receptors and to other TRAF proteins. In addition TRAF2-TRAF7 proteins have Ring and Zinc finger motifs in their N terminus that are important for signaling downstream events.
Knock out mice on TRAFs genes were established (Reviewed by Bishop 2004, Bradley 2001, and Chung 2002). TRAF2 knock out die prematurely, show no TNF-mediated INK activation in fibroblasts. They show elevated serum TNF levels and increased sensitivity to TNF induced death in thymocytes and fibroblasts. In addition, they have B cells impaired in the TNF and CD40 induced canonical NF-κB activation. Also, they show deficient CD40 induced TRAF3 degradation and constitutive alternative NF-κB activation in B cells. TRAF3 knock out show deficient in all lineages of peripheral leukocytes. They show defective isotype switching in response to T-dependent antigens and LMP1 signaling defective in B cells.